This invention relates to methods for removing sulfur oxides from waste gases. More particularly, this invention relates to electrodialytic methods for removing sulfur oxides, e.g., SO.sub.2 and SO.sub.3, from waste gases recovering SO.sub.2, and regenerating the process solution. A completely closed-loop electrodialytic method suitable for continuously removing sulfur oxides from sulfuric acid tail gases, recovering SO.sub.2 for use in a sulfuric acid plants, regenerating the process solution and improving the water balance in a sulfuric acid plant is also contemplated.
Environmental considerations have placed stringent limits on emissions of sulfur oxides from stationary sources, e.g., utility plants and chemical plants. In response to the emissions limits, many processes have been developed which use alkaline scrubbing agents to remove sulfur oxides from waste gases from these plants. These processes include lime and limestone scrubbing, magnesium oxide scrubbing, sodium scrubbing with thermal regeneration (e.g., see U.S. Pat. Nos. 3,477,815 and 3,485,581,) sodium scrubbing with electrolytic regeneration [e.g., see U.S. Pat. Nos. 3,475,122, 3,523,880 and K. A. Meliere et al., "Description and Operation of the Stone & Webster/Ionics SO.sub.2 Removal and Recovery Pilot Plant at the Wisconsin Electric Power Co. Valley Station in Milwaukee", U.S. EPA Report No. 650/2-74-1266 (1974)], ammonium scrubbing and electrodialytic concentration (e.g., U.S. Pat. Nos. 3,974,258, and 4,041,129) and catalytic oxidation processes. Other processes have been reviewed by A. V. Slack of Noyes Data Corp., Park Ridge, N.J. ["Sulfur Dioxide Removal from Waste Gases" Pollution Control Review No. 4 (1971)], and N. Kaplan and M. A. Maxwell "Removal of SO.sub.2 from Industrial Waste Gases", Chemical Engineering, Vol. 84, No. 22, p. 127-135 (Oct. 17, 1977).
The Welman-Lord process has been developed for use at both power plants and sulfuric acid plants. See "Welman-Lord SO.sub.2 Recovery Process--Flue Gas Desulfurization, U.S. EPA Report No. 625/2-77-011 (1977). This process produces a sodium bisulfate liquor which is treated to yield SO.sub.2 and sodium sulfate. However, the treating step a costly vacuum evaporation step to convert sodium bisulfite liquor into SO.sub.2 and sodium sulfate.
Electrolysis, as disclosed in U.S. Pat. No. 3,475,122, 3,523,880, 3,974,258 and 4,041,129, is not a very efficient process. For the production of acid and base from salt and water, electrolytic processes generate H.sup.+ and OH.sup.- ions at the electrodes only and, at the same time, generate H.sub.2 and O.sub.2 (or other electrode oxidation and reduction products). Thus, each equivalent of H.sup.+ and OH.sup.- generated results in an equivalent amount of H.sub.2 and O.sub.2 (or other oxidation and reduction products) being produced. This significantly reduces the rate of production of acid and base.
Electrodialytic water splitting, on the other hand, generates H.sup.+ and OH.sup.- ions from each of several bipolar membranes positioned between the electrodes without forming H.sub.2 and O.sub.2 (except for the relatively limited quantities of H.sub.2 and O.sub.2 at the electrodes which are the only places where oxidation-reduction is taking place). Therefore, oxidation-reduction products (H.sub.2 and O.sub.2) are formed in only small amounts relative to the total amount of H.sup.+ and OH.sup.- formed. Thus, the process of electrodialytic water splitting requires less energy than an electrolysis process to produce acid and base from salt and water.
Recently, electrodialytic water splitting processes have been employed to remove SO.sub.2 from water gases. See U.S. Pat. Nos. 4,082,835 and 4,107,015. Both processes involve the production of sodium bisulfite liquor which is treated to produce SO.sub.2 and sodium sulfite. However, the sodium sulfate produced by the processes from the oxidation of SO.sub.2 to SO.sub.3 must be purged from the system and treated separately.